It is known in the art to vibrate objects at specific frequencies to measure mechanical factors such as rigidity.
It is also known that vibration of fruit can indicate a state of ripeness or non-ripeness. See for example U.S. Pat. No. 2,277,037.
It is also known that solid fruit transmits higher frequency vibrations with greater efficiency and that, for a given fruit sample, the natural or resonant frequency decreases as the fruit matures or begins to degrade.
It is also known that many fruits exhibit multiple natural frequencies and that only some of these frequencies contain information which relates to fruit texture, see: J. A. Abbott et al "Sonic Techniques for Measuring Texture of Fruits and Vegetables", Food Technol. 1968, 22 (5):101-112; E. E. Finney et al, 1978 "Methods for testing the dynamic mechanical response of solid foods", J. Food Quality 2:55-74, and; E. E. Finney, Jr. 1972, "Vibration Techniques for Testing Fruit Firmness", J. Texture Studies, 3:263-283. The prior art devices such as those described by: E. E. Finney, "Mechanical Resonance within Red Delicious Apples and Its Relation to Fruit Texture", Trans. ASAE, 1970, pages 177 to 180, H. L. Clark et al, 1942, "Fruit Ripeness Tester" U.S. Pat. No. 2,277,037, and J. A. Abbott et al 1968, Ibid; employ a frequency scanner to vibrate the object at a series of frequencies and query each frequency to detect the natural frequency for that sample. This method is time consuming and can be damaging to the fruit. Other devices excite the fruit (see e.g. D. R. Bower et al, "Application of Vibrational Sorting to Blueberry Firmness Separation", Trans. ASAE, 1976, pages 185-191) with a single frequency and determine how that sample's vibration compares with the calibration set. This method can be inaccurate if the fruit's mass or variety differs from that of the calibration set. Furthermore, the number of ripeness categories into which a fruit may be designated is highly limited by the single frequency method. Still other devices such as Abbott et al supra require that fruit be suspended in space by the stem end with vibrations probed about the fruit's equator. This method is time consuming, and the suspension is impractical for field operations. Furthermore, this method is highly sensitive to air currents and background vibrations in the environment.
It is also known from Finney 1970, supra, that devices used to couple vibrations from the vibration generator to the commodity must be capable of transmitting the vibrations without distortion. Prior art devices have employed rigid frames for this purpose which damage fruit and do not reliably excite the same resonant modes of vibration in consecutive samples, thus promoting improper classification of the fruit.
The present invention takes advantage of the fact that, as agricultural commodities mature, internal changes take place in regard to the rigidity of the cell walls, the stiffness of the cell walls and intercellular bonding agents, the turgor pressure within the cells and the size of intercellular air spaces. Furthermore, as these changes occur during maturation or degradation, the elastic nature of the agricultural commodity will change. Mechanical, high frequency vibrations are transmitted more efficiently through a less elastic material. A less elastic material will also develop its own natural vibrations at higher frequencies when excited. A sonic impulse containing regulated frequencies can be passed through an agricultural commodity to excite and identify these natural vibrations as a measure of the internal condition (i.e. texture, maturity and extent of damage (if any)) of the whole agricultural commodity. This method of analysis provides a full spectrum of sonic wave energy that has passed through the agricultural commodity, which can then be classified according to a set of calibrated samples. The output is a measure of the internal condition of the whole agricultural commodity. Utilization of the condition measurement of the present invention may reduce agricultural commodity loss and increase net return by providing an objective measure upon which to channel product flow to the market where it may be utilized most efficiently and economically. For example, in 1988, the United States produced 192 million bushels of apples, of which 111 million were marketed fresh and 80 million were processed. Another half million were not marketed. Greater returns can result when losses are reduced and specific levels of consumer quality can be guaranteed, particularly in the dominant fresh market. Furthermore, producers can benefit from early evaluation of harvest quality.